Schulte (U.S. Pat. No. 5,763,886) teaches a two-dimensional imaging backscatter probe for using a source of gamma-rays to illuminate a surface and for generating a two-dimensional image of the backscattered radiation. It is valuable, in many applications, to know the shape and volumetric distribution as well as material characteristics of objects lying behind or beneath the illuminated surface. Schulte, however, fails to suggest that any depth or compositional information may be obtained with respect to objects lying behind or beneath the illuminated surface or to teach any manner in which such information may be obtained. Additionally, Schulte requires that the probe be moved and that the position of the radiation detectors with respect to the target be sensed using means external to the probe in order to map the backscattered radiation.